Demand for bandwidth in lightwave communication systems is ever increasing. This increasing demand can be met by installing additional fiber cable. However, augmentation of existing long distance fiber systems is costly. Thus, it is desirable to use existing systems to maximum capacity.
One technique for increasing the capacity of an existing fiber system is a form of space division multiplexing in which two optical signals of different polarizations are transmitted through a single optical fiber. Transmission of different data sequences in the same fiber via a separate polarization for each sequence permits more information to pass through the fiber without increasing the bit rate in any particular polarization. But, it is difficult to discriminate between two signals of different polarizations because polarizations that are easily distinguishable when the signals are transmitted become indistinguishable when the signals are received because of polarization rotation and distortion. For example, polarization mode dispersion and nonlinear effects, such as cross-phase modulation and four-photon mixing, induce phase changes and power dependent distortion in the polarized optical signals as the signals pass through the fiber. This, in turn, makes the two interfering optical signals difficult to distinguish from each other. Inability to distinguish between the signals arises because components of the optical signal in one polarization are sufficiently distorted to appear in the optical signal of the other polarization and vice versa. There is no mechanism within prior art receivers to intelligently extract unwanted components from each received optical signal so that the original data signals can be separated during recovery.
Special receivers equipped with polarization controllers, such as lithium niobate (LiNbO.sub.3) polarization controllers or mechanically adjustable bulk optical components, were used in the prior art to correct polarization rotation experienced by an optical signal. Polarization controllers adjusted an incoming arbitrary polarization to match the needed polarization at the receiver. However, receivers using polarization controllers are complex and reliability is a concern. Polarization controllers also are slow to adjust to different polarizations. For example, mechanically adjustable components are physically rotated and repositioned to detect different polarizations. Lithium niobate polarization controllers require the application of large voltages to detect different polarizations. Moreover, dispersive and nonlinear effects for fibers and the differences in these effects on the different axes of the fiber cause optical signals transmitted in separate "non-interfering" polarizations to suffer mutually induced distortion and cross-talk, thereby making separation and extraction of data sequences modulated on the optical signals unwieldy at best and perhaps even impossible when the cross-talk is too high. Thus, a polarization controller may be able to adjust the polarization of one of the two optical signals to correct polarization distortion in that signal, but the polarization controller may not correct polarization distortion in the second signal and cannot reduce cross-talk between the first and second signals. As a result, space division multiplexing using different polarizations is difficult and may not be practically realizable in many cases using present receiver and polarization controller technology.